The present invention relates to a gap voltage detector for use in a wire-cut electric discharge machine, which is capable of accurately detecting a gap voltage between a wire electrode and a workpiece.
In a wire-cut electric discharge machine, it is known to effect appropriate control of electric discharge machining on the basis of a detected value of a gap voltage between a wire electrode (hereinafter referred to as wire) and a workpiece. Conventionally, a voltage between the workpiece and a conductor disposed above or below the workpiece for connecting a machining power source with the wire, is detected as the gap voltage. However, the thus detected voltage includes voltage drop components attributable to a wire resistance between the conductor and the workpiece and an inductance therebetween. As a position at which electric discharge occurs between the wire and the workpiece varies, a variation occurs also in these voltage drop components. In particular, a large variation in these components is found when the workpiece has a large thickness. As explained above, the detected voltage includes variable components attributable to a variation in the electric discharge position, and hence cannot indicate an accurate gap voltage.
Further, it is also known to subtract a correction value representative of a voltage drop component attributable to an inductance between the conductor and the workpiece from the detected voltage, so as to correct the detected voltage, as disclosed in Japenese Provisional Patent Publication No. 61-180719. However, even in this case, the detected voltage thus corrected cannot indicate an accurate gap voltage because the just mentioned correction value does not reflect a variation of the voltage drop component. Also, the gap voltage cannot be detected accurately, in the case of effecting correction for voltage drop component associated with the wire resistance.